A rapidly-developing and acute form of transneuronal degeneration has been found to occur during a certain period in the development of the chick visual system. We propose that this can serve as a valuable model for further study of neuronal trophism because it demonstrates a particularly severe transneuronal reaction, because it occurs only during a restricted period of development, and because the source of afferent axons, the eye, is so accessible for chemical or surgical manipulations. Accordingly, we propose to further define this phenomenon and to initiate experiments designed to explore the trophic phenomenon. First, we wish to study the effects of embryonic enucleation on the growth of tectal neurons and to determine whether the acute degeneration we have observed is permanent or is instead a transient event akin to chromatolysis. We also will perform experiments to see if any portion of the fiber pathways undergoing transneuronal change persist or perhaps regenerate after long survivals. Second, we wish to study the ultrastructure of transneuronal degeneration in retinal terminals. Third, by injecting inhibitors of either protein synthesis or axonal transport into the eye, we hope to provide some indication of whether or not a trophic factor may be passing from eye to tectum. Fourth, by culturing tectal neurons in vitro in the presence of retinal explants or homogenates of retina, we hope to demonstrate influences on growth of the tectal cells, which would provide further indication of a trophic relationship.